Red rot is a type of leather deterioration. Leather books suffering from red rot are powdery and red-brown. Not much can be done to treat this condition (oiling does not help, but only makes more of a mess). One possible option is to put a protective cover on the book to keep the deterioration more intact.
Because of staff levels and cost constraints, it is best generally to only send books that are circulating to the Preservation Unit that have this condition if that would be the only reason for sending them.